2 Principal Features of the Sky 2.1. Star Patterns: Asterisms the absence of stars, the “dark constellations.”2 Chinese con- stellations were different from and far more numerous than and Constellations were those of the Mediterranean area. As far as we are aware, the oldest extant Chinese star chart on paper is con- 2.1.1. Stellar Pattern Recognition tained in a 10th-century manuscript from Dunhuang, but there is far older evidence for sky charting from this area of About 15,000 stars are detectable by the human eye, most the world (see §10 and §2.2.3); a compilation by Chhien Lu- of them near the limit of visibility. At any one time, we may Chih listed 284 constellations containing a total of 1464 stars be able to see a few thousand stars in a dark sky, but we tend and is said to be based on a Han catalogue (see §10.1.2.3; to remember only striking patterns of them—asterisms and Yi, Kistemaker, and Yang (1986) for new maps and a such as the Big Dipper or whole constellations such as Ursa review of historical Chinese star catalogues). Major (the Big Bear) or Orion (the name of a mythological Western constellations in current use largely derive from hunter)—and so it has been for millennia. Today, the entire ancient Mediterranean sources, mainly the Near East and sky has been divided into constellations; they are not defined Greece, as we show in §7. The earliest surviving detailed according to appearance alone but according to location, description of the Greek constellations is in the poem and there are no boundary disputes. The modern names and Phaenomena by the Greek poet Aratos (Aratus in the locations are more or less those of Argelander (1799–1875) Roman sources), ~250 b.c. (Whitfield 1995, p. 23). The con- for the Northern Hemisphere and John Herschel stellations portrayed in the poem derive from a work also (1824–1896) for the Southern, but the present divisions of called Phaenomena, which has not survived, by the Greek the constellations1 were adopted by the International Astro- astronomer Eudoxos (or Eudoxus) (4th century b.c.). One nomical Union (IAU), the chief authority on such matters of the later sources that discusses this work is that of the sole as astronomical nomenclature, in 1930. The IAU has estab- remaining manuscript of Hipparchos (~150 b.c.), one of the lished 88 constellations in the sky; many reflecting an ancient greatest astronomers of antiquity. Many of the constellations heritage. can be seen as raised images on the Farnese Globe, the The names of the constellations recognized in antiquity oldest extant celestial globe, dated to the 2nd century b.c., were based on but representing a copy of an older work. Aratos mentioned • Mythological figures 47 constellations, whereas Claudius Ptolemy (~150 a.d.), • Animals or inanimate objects as perceived in the sky the source of much of our knowledge about Hipparchos, • Geographical or political analogues referred to 48 in the major astronomical work that we know • Associations with seasonal phenomena, or some other today as the Almagest. basis In ancient Greek usage, the constellations were the figures. For example, in the constellation of Cassiopeia, As we will show in later chapters, non-Western traditions the star z Cassiopeiae (abbreviated z Cas) is described as have perceived a rich variety of star patterns; some include “the star on the head”; a Cas, as “the star in the breast”; and 1 Boundaries are along coordinates of right ascension and declination referred to the equinox of 1875.0. See sections below for explanations of these terms. 2 See §14.2.5, for a Peruvian example. D.H. Kelley, E.F. Milone, Exploring Ancient Skies, DOI 10.1007/978-1-4419-7624-6_2, © Springer Science+Business Media, LLC 2011 9 10 2. Principal Features of the Sky h Cas as “the star over the throne, just over the thighs.” In constellation of Ursa Minor, the Little Bear, and the Perseus, the variable star Algol (b Per) is described as the Pleiades is a well-known asterism in the constellation “bright one” in the “Gorgon’s head.” Not all naked eye stars Taurus, the Bull. An exception is the Summer Triangle, com- fitted neatly into these groupings, so many stars were posed of the bright stars Vega, Deneb, and Altair in the con- omitted from the constellations. Those outside the accepted stellations Lyra, Cygnus, and Aquila, respectively. Even a figures were referred to as “unformed” (amórfwtoi; our single star may constitute an asterism. The star Spica, for word “amorphous” derives from a related word), or “scat- example, the brightest star in the constellation of Virgo, has tered” (spordeV, related to the Greek word for seed, been envisaged as a spike of wheat. spor, broadcast during sowing, and our cognate word, Modern common names of naked eye stars, derive from “sporadic”). The IAU reorganization created constellation European and Arabic usage, as well as proper names devised “homes” for these “unformed” stars. by Johann Bayer in 1603. The Bayer designations use lower- case Greek letters and, after these are exhausted, small Roman letters, to identify stars in a given constellation, for 2.1.2. Star Charts example, u Herculis or i Bootis. When these were exhausted, capital Roman letters were used. The lettered type of desig- The depictions of the Greco-Roman constellations as they nation was later extended to the Southern Hemisphere by were known in Ptolemy’s time (~150 a.d.) were preserved Nicolas Louis de Lacaille (1763) and John Herschel (1847). in Arabic sources, one of the best known being that of the The Greek letters are universally accepted, but an alterna- astronomer al-Sufı (10th century). R.H. Allen (1963) states tive designation to the Bayer letters for the fainter stars is that the sky representations of post-Renaissance Europe that of the Flamsteed numbers (Flamsteed 1725, Vol. 3), as, derive from those of Albrecht Dürer (1471–1528) of 1515 for example, 44 Bootis = i Bootis. Giuseppe Piazzi (1803) (Figure 1.1), in which the star positions from Ptolemy’s also published star catalogues in 1803 and 1814 (see catalogue were set down by another resident of Nürnberg Piazzi/Foderà Serio 1990). The Flamsteed numbers increase (Nuremberg), a mathematician named Heinvogel. The posi- with right ascension, a coordinate that increases from west tions were subsequently improved and more stars added, but to east (see §2.2.3). Many catalogues of stars and other the figures of Dürer essentially remained the same through objects use positional or sequence numbers, usually increas- the charts of Bayer (1603), Flamsteed (1729), and Arge- ing with right ascension. The best known star catalog of this lander (1843). More details about star charts from 1500 to kind is the Bright Star Catalog (Hoffleit 1982), which uses 1800 can be found in Warner (1979), and an even wider the positional sequence numbers of the Harvard Revised range of charts is found in Stott (1991/1995) and Whitfield Photometry Catalog (Pickering 1908); thus, BS 7001 = HR (1995). 7001 =aLyrae. The representations of the more obvious asterisms dif- Usually, the Greek letter designates the relative bright- fered widely from culture to culture. A familiar example is ness of the star within the constellation, but occasionally the Big Dipper, still known in England as the plough, and in they were assigned to a positional sequence, as in Ursa Germany and Scandinavia as the Wagen (wagon). In the Major. In the list of modern constellations, Table 2.1, the star Roman republic, it was the plow oxen. On many pre-19th- names are in Latin, with the historically earliest names refer- century maps and star charts, the term Septentrion or some ring to Latin forms of Greek originals. The columns contain variety of this term appears. The expression became syn- both nominative and possessive3 cases of the names, English onymous with the North, or northern regions, but originally equivalents, notable stars and other objects, and both meant the seven plow oxen. R.H. Allen (1963) says that the modern and ancient asterisms that are within the modern Big Dipper was known as a coffin in parts of the Mideast, boundaries. Only a few objects that cannot be seen unaided a wagon or bear in Greece, and a bull’s thigh in pre- under clear and dark sky circumstances are included. Hellenistic Egypt. Systematic attempts were made to “Double stars” are stars that appear close to each other rename the constellations at various times. Giordano Bruno in the sky; sometimes they are indeed physically close to (1548–1600) sought to invest the sky with figures represent- each other and interact gravitationally, but not always. The ing Moral Virtues. Julius Schiller of Augsburg produced the pair of stars Mizar and Alcor (z Ursae Majoris and 80 Ursae most widely known type of Bible-inspired charts in 1627. Majoris, respectively), in the handle of the Big Dipper, is an R.H. Allen’s (1963) encyclopedic search into the origins of example of a naked-eye double. star names and constellations reveals several other Euro- Types of “variable stars” are named after their prototypes, pean attempts to recast the constellations, although the such as delta Cephei or RR Lyrae. In the Bayer designations, various sources used by him are not always treated critically. no visible star had been assigned a letter later in the alpha- bet than Q; consequently, Argelander suggested that desig- 2.1.3. Modern Nomenclature nations of R and later would be used solely for variable stars. This scheme has been followed dogmatically to a logical Today, constellations refer to specified areas on the celestial conclusion ever since.